1. Field of Invention
The invention generally relates to the field of earth-boring (drilling) technology. With more particularity the invention is designed to be used in combination with downhole signal generators which are used to acquire "real time" downhole data during drilling and transmit same to the surface for interpretation (herein called "MWD", including, but not limited to, tools which transmit data by means of mud pulses and tools which transmit data by electric line, sometimes referred to as steering tools). The invention relates to means to centrally stabilize MWD tools during drilling yet provide convenient means for deployment and removal of MWD tools downhole without the necessity of removing or reinserting the drill string from the hole.
2. General Background
In the art of earth-boring MWD represents an improvement over previous drilling process. MWD allows for the surface acquisition of downhole data during drilling, thereby reducing the need for costly and time consuming drill string tripping and logging/survey runs otherwise necessary to acquire downhole data. Further, the acquisition of "real time" data during drilling can be of substantially greater value than later acquired data. MWD can be used to monitor downhole temperature and pressure, monitor formation properties, monitor bit weight and torque, control direction of the well, detect abnormally pressured formations, evaluate potentially productive formations and monitor/evaluate other important downhole conditions.
MWD systems typically include the placement of a complex and expensive self-contained package of sensors, encoders, power supplies and transmitters immediately above, or very near, the drill bit. While this position is desirable for sensing the variables in question, it presents a harsh, hot, highly pressured, dirty and high shock load environment for the MWD tool. MWD tool failures are not uncommon, requiring retrieval and replacement of the tool downhole. In the event the drill pipe becomes stuck in the hole the MWD tool may be permanently lost.
Early MWD tools were typically made up as an integral part of the bottom hole assembly. However, this arrangement is not ideal. In the event of MWD tool failure the entire drill string had to be tripped out and back in the hole to replace the MWD tool. Further, in the event of loss of the bottom hole assembly due to twist off or sticking the expensive MWD tool was lost. Thus retrievable MWD systems were developed.
A retrievable MWD system utilizes a passive receptacle which is run into the wellbore as part of the bottom hole assembly. Contained within this receptacle is a removable MWD electronics package which can be retrieved through the drill string. In the event of a tool failure, downtime is minimal because the electronic components can be retrieved and replaced by wireline (or coiled tubing), eliminating the time consuming necessity, of tripping the entire drill string. If the drill string itself were to become permanently stuck, the expensive MWD electronics package may be retrievable prior to abandonment of the bottom hole assembly.
It is well documented that downhole vibration can be severe during the drilling process. One study, SPE/IADC 16109, found that bottom hole assemblies can be subjected to lateral shocks in excess of 200 g's, and axial vibrations of up to 3.5 g's during drilling operations. While these vibrations can have a harmful effect on almost any downhole equipment, the shocks are especially damaging to the sensitive electronics of MWD systems in general, and retrievable MWD systems in particular. As these downhole vibrations act on retrievable MWD tools, the removable components have a tendency to rattle or bang against the internal walls of the surrounding MWD receptacle thereby amplifying vibrations. Thus, to reduce the potential damage caused by these downhole shocks, it is desirable to centralize and secure the inner electronic components of retrievable MWD systems within their surrounding MWD receptacles.
Currently, one common method of improving the centralization of retrievable MWD tools involves the use of donut shaped rubber rings. These rubber rings are placed around the circumference of retrievable MWD tools to increase their outer diameter and at cushion some of the shock effects described above. However, this method is not ideal. Because the inner MWD components are designed to be retrieved through the drill string, the tool's largest outer diameter must always be smaller than the smallest drill string restriction; otherwise, a tool could not pass through the drill string restriction on its way to the surface. Accordingly, the rubber rings used to centralize retrievable MWD tools must also be sized to pass through the smallest drill pipe restriction which will be encountered. This sizing limitation results in a less than tight fit between the retrievable MWD tool and the surrounding MWD receptacle, which hinders the effectiveness of the centralizer rings. Although the rubber centralizer rings provide some benefit, the lack of a tight fit still permits some amplification of the downhole shocks on the retrievable MWD tool. Therefore, the need exists for a centralizer which can pass through relatively small drill string restrictions, but which can also rigidly centralize and secure a retrievable MWD tool from movement within a downhole MWD receptacle.